1. Field of the Invention
This invention relates to rotor blade devices. More specifically, the invention is a method and a system for controlling vortices produced by rotating blades such as those found on rotorcraft.
2. Description of the Related Art
Significant noise and efficiency problems are caused by the vortices that trail in the wakes of rotary wing aircraft or rotary bladed equipment. For example, on rotary wing aircraft (also known as “rotorcraft”), a major source of noise and vibration arises from the interaction of rotor blade tip-vortices with succeeding blades, the airframe, empennage and tail rotor of the vehicle. In particular, blade-vortex interaction (BVI) is responsible for most of the sound pollution caused by helicopters, particularly during low-speed landing approach and maneuvers. Tip flows also present significant problems for rotary bladed equipment such as turbomachinery (e.g., axial compressors, fans and blowers). As much as 30% of the losses in a turbomachine can be attributed to the clearance flow near the tip of the rotating element. In addition, tip-clearance induced noise, both in the form of rotor self noise and rotor-stator interaction noise, constitutes a significant component of total fan noise.
With respect to rotorcraft, there are two main approaches for BVI noise reduction. The first approach involves the passive or active reduction in vortex intensity or strength while simultaneously increasing vortex core size. Passive devices such as spoilers, winglets and vortex generators have been proposed, while active methods such as surface and tip blowing have also been investigated. One of the key problems associated with these current devices and methods is that they invariably result in a degradation of overall rotorcraft performance. For example, passive devices such as spoilers increase drag. Surface blowing reduces blade loading while tip blowing requires large power supplies to supply blowers.
The second approach to BVI noise reduction involves increasing the distance between the vortex and the rotor blade thereby minimizing the interaction. Techniques proposed include the use of active flaps, higher harmonic control of blade pitch, or individual blade control. In these cases, reductions in BVI noise have been reported, but the inputs that are required to drive the BVI noise reduction systems can themselves promote and increase vibrations.